Three people were injured in a knife attack at a Paris railway station early on Saturday.
Police said they arrested a suspect at the Gare de Lyon after he was tackled by a security guard.
The suspect's motive was unclear, but Paris police chief Laurent Nunez said it did not appear to be a terrorist attack.
The man was carrying residency papers from Italy and may have had mental health issues, Mr Nunez said. Reports said he was a national of Mali.
One victim was seriously hurt but not in a life-threatening condition, police said. Two suffered slight wounds.
Forensic investigators were examining a hammer and knife used in the attack.
Posting on social media, Interior Minister Gerald Darmanin described the attack as an “unbearable act”.
Security in Paris is being ramped up as it prepares to welcome 10,500 athletes and millions of visitors for the summer Olympics.
The games are to open with a massive open-air ceremony along the River Seine on July 26, a major security challenge in a city that has been repeatedly hit by terror attacks, most notably in 2015.
Most recently, a man attacked passers-by near the Eiffel Tower in December, killing a German tourist with a knife and injuring two others.
